1. Used by people who cannot type full sentances with proper grammar.
2. Oh my god.
3. O mah gawd.
4. When used by people with grammar skills, it is often in mocking.
Defintion 4: Omg. AHAHAH. Anyway...
Definition 1: OmG dId U c WaT hApEnEd LaSt WeDnSdAy!!!!!11111